The present invention relates to a dielectric filter which comprises a plurality of resonators disposed side by side and is suitable for forming input/output terminal electrodes on a side surface.
There have hitherto been proposed various types of dielectric filters, in which terminal electrodes are formed on a side surface perpendicular to an open end face of a dielectric porcelain block 42 (i.e., an exposed-dielectric surface having conductors exposed thereon).
Terminal electrodes formed on the side surface perpendicular to an open end face of the dielectric porcelain block 42 are soldered to another circuit board and act as I/O terminals of the dielectric filter. Another electrode placed on the same plane where the terminal electrodes are provided acts as a ground electrode.
Commonly-used terminal electrodes include a terminal electrode formed by means of printing and sintering conductive material, and a terminal electrode formed by means of plating (see, for example, Japanese Patent Publication 8-307119/(1996) and 9-260903/(1997)). The terminal electrode assumes a structure such as that shown in FIG. 13.
In a dielectric porcelain block 2 of the dielectric filter 1 shown in FIG. 13, a plurality of resonators 10 are provided side by side, wherein each resonator 10 is formed by coating the interior surface of a through hole 11 with an interior conductor 12. I/O terminal electrodes 20 are provided on a side surface 4 perpendicular to an open end face 3 of the dielectric porcelain block 2 having the through holes 11 formed therein. The remaining side surfaces and bottom (i.e., a surface opposite the open end face 3) of the dielectric porcelain block 2 are covered with an exterior conductor 22 such that the I/O terminal electrodes 20 are isolated from the exterior conductor 22 by means of an exposed-dielectric section 21.
There have also hitherto been proposed a method of forming independent I/O terminal electrodes, by means of metallizing the entire side surfaces of a dielectric porcelain block; forming recesses in one of the side surfaces by means of sandblasting, laser, or etching; and masking electrodes with resist before the dielectric porcelain block is metallized.
As is evident from an enlarged portion shown in FIG. 13, the terminal electrode 20 and the ground electrode (i.e., the exterior conductor 22) are higher than the exposed dielectric porcelain block 2 located therebetween by an amount corresponding to the thickness of the electrodes.
The terminal electrodes 20 are located at the highest position above amount surface of the dielectric porcelain block 2. When a dielectric filter is soldered to another circuit board by use of cream solder, the dielectric filter is mounted on a circuit board by means of squeezing cream solder, since no gap exists between a circuit board and a terminal electrode. Accordingly, if the amount of solder is greater than appropriate, a solder bridge arises between adjacent terminals, thus causing a short circuit. Occurrence of a short circuit will be described specifically by reference to FIGS. 14A through 14C.
FIGS. 14A through 14C show an example solder bridge arising in a dielectric filter having a related-art terminal electrode. As shown in FIG. 14A, a cream solder layer 33a is applied over an electrode 32a, and a cream solder layer 33b is applied over an electrode 32b. When the dielectric porcelain block 2 is mounted on the electrodes 32a and 32b laid on a circuit board 31 and is pressed against the electrodes 32a and 32b, the cream solder layers 33a and 33b squeeze out in a manner as shown in FIG. 14B, because no gap exists between a terminal electrode 20, a ground electrode (i.e., an exterior conductor 22), and a circuit board 31 of the dielectric porcelain block 2.
So long as a sufficient space exists between the electrodes 32a and 32b, occurrence of solder bridges between the electrodes 32a and 32d can be prevented. However, in a high-frequency dielectric filter which is to be used for higher frequencies and miniaturization, an interval between electrodes becomes narrower, and a short circuit due to solder bridges becomes apt to arise between the electrodes 20 and 22, as shown in FIG. 14C.
In connection with a dielectric filter, a dielectric porcelain block per se is greater in area than electrodes. Hence, the strength of a connection must be enhanced by means of increasing the area of a place to be soldered, and the amount of solder to be used must be increased. An increase in the amount of solder eventually results in ease of occurrence of a short circuit.
When terminal electrodes and a ground electrode of a dielectric filter are formed, by means of electric plating, areas where terminal electrodes are to be formed assume island-shaped geometries, and as a result plating of the terminal electrodes becomes thinner. A pattern is formed by means of resist, and hence variations may arise in positions where terminals are to be formed, depending on the precision of printing of resist, thereby adversely affecting a filtering characteristic of the dielectric filter.
In a case where electrodes are formed by means of plating, a plating bridge (elongation of plating) may arise between a terminal electrode 20 and a ground electrode (i.e., an exterior conductor 22) formed on the dielectric porcelain block 2 of the dielectric filter 1, thereby resulting in a short circuit such as that shown in FIG. 15. After removal of resist, the bridge still remains, thereby inducing a short circuit.
When molded bodies which are to become dielectric porcelain blocks are sintered, a maximum number of molded bodies are put into a furnace as close to each other as possible. If contact of large area arises between molded bodies, the molded bodies will stick to each other after sintering, thus resulting in occurrence of defectives.
The present invention is aimed at obviating drawbacks of a dielectric filter having a related-art structure and at providing a dielectric filter which is easy to mount and provides a high manufacturing yield and stable quality, as well as a method of manufacturing the dielectric filter.
Other objects and novel features of the present invention will be manifest by reference to embodiments to be described later.
To this end, the present invention provides a dielectric filter formed from a dielectric block and a plurality of resonators formed in the dielectric block side by side, each resonator being formed by coating an interior surface of a through section with an interior conductor, wherein, on a side surface of the dielectric block perpendicular to an open end face having the through sections formed therein, there are formed protrusions having a height lower than that of the through sections.
By means of such a construction, when terminal electrodes are provided on a side surface of a dielectric block, dielectric protrusions are located between the electrodes. Hence, when the dielectric porcelain block is mounted on a circuit board by means of cream solder, the amount of solder which enters the exposed dielectric of the protrusions from electrodes becomes less, thereby eliminating squeezing out of solder to adjacent electrodes. Consequently, there can be prevented occurrence of a short circuit, which would otherwise be caused by solder bridges.
One or more protrusions are provided on at least one of the side surfaces of the dielectric porcelain block during molding. As a result, there can be prevented occurrence of contact between molded bodies over a wide area. Consequently, there can be prevented failures, which would otherwise be caused when molded bodies come into contact with each other over a wide area during sintering.
Preferably, each of the protrusions is formed so as to spread across two or more side surfaces.
By means of such a construction, a terminal electrode can be provided at a corner of the dielectric block.
Preferably, a plurality of protrusions can be formed on the same plane.
By means of such a construction, a plurality of terminal electrodes can be provided on a single plate of the dielectric block.
Preferably, terminal electrodes are formed such that the protrusions are located as a boundary between the electrodes.
By means of such a construction, since protrusions have been formed on a side surface beforehand, the positional accuracy of the terminal electrodes can be improved, thereby diminishing variations in the filtering characteristic of a dielectric filter.
Preferably, the protrusions are located at least between adjacent terminal electrodes.
By means of such a construction, when terminal electrodes are provided on a side surface of a dielectric block, dielectric protrusions are located between the electrodes. Hence, when the dielectric porcelain block is mounted on a circuit board by means of cream solder, the amount of solder which enters the exposed dielectric of the protrusions from electrodes becomes less, thereby eliminating squeezing out of solder to adjacent electrodes. Consequently, there can be prevented occurrence of a short circuit, which would otherwise be caused by solder bridges.
Preferably, the protrusions are provided in a position other than the boundary between the terminal electrodes.
By means of such a construction, there can be prevented failures, which would otherwise be caused when molded bodies come into contact with each other over a wide area during sintering. Thus, the construction is effective for improving molding yield and useful for stabilizing the position of the dielectric filter when the filter is mounted to a circuit board.
Preferably, the extent to which the protrusions are to protrude from a side surface of the dielectric block is greater than the thickness of an exterior conductor formed on the side surface.
By means of such a construction, when the dielectric porcelain block is mounted on a circuit board by means of cream solder, the protrusions projecting from the exterior conductor prevents squeezing out of solder to adjacent electrodes. Consequently, there can be prevented occurrence of a short circuit, which would otherwise be caused by solder bridges.
Preferably, the interior and exterior conductor layers are formed by plating.
By means of such a construction, the terminal electrodes, the interior conductor, and the exterior conductor can be formed simultaneously by means of plating.
The present invention also provides a method of manufacturing a dielectric filter formed from a dielectric block and a plurality of resonators formed in the dielectric block side by side, each resonator being formed by coating an interior surface of a through section with an interior conductor, the method comprising the steps of: molding the dielectric block such that protrusions having a height lower than that of the through sections are formed on a side surface of the dielectric block perpendicular to an open end face having the through sections formed therein; coating the dielectric block including the surfaces of the protrusions with a conductor layer; and abrading tops of the protrusions, thereby causing the surface of the dielectric block to become exposed and forming electrodes defined by the protrusions.
By means of such a construction, when terminal electrodes are formed by means of plating, top surfaces of protrusionsxe2x80x94which have been formed on a side surface beforehand after the entire surface of a dielectric porcelain block exclusive of an open end face has been platedxe2x80x94are abraded, thus eliminating the conductor layer from the top of each of the protrusions and forming terminal electrodes defined by the protrusions. The number of steps of forming a resist mask on a surface on which terminals are to be formed can be reduced. Further, protrusions are formed between terminal electrodes so as to constitute a boundary between the terminal electrodes, thereby preventing occurrence of a short circuit, which would otherwise be caused by elongation of plating. Moreover, the positional precision of electrodes can also be improved.
When the entire surface of a dielectric porcelain block is plated after resist mask has been formed over an open end face according to the related-art method, terminal electrodes become island-shaped floating electrodes. When the dielectric porcelain block is electrically plated, a reduction arises in the thickness or strength of plating. Since the present method prevents occurrence of floating electrodes, the foregoing problems do not arise. Thus, there can be formed terminal electrodes having stable strength and thickness.
The present invention also provides a method of manufacturing a dielectric filter formed from a dielectric block and a plurality of resonators formed in the dielectric block side by side, each resonator being formed by coating an interior surface of a through section with an interior conductor, the method comprising the steps of: forming the dielectric block by means of injection molding, such that protrusions having a height lower than that of the through sections are formed on a side surface of the dielectric block perpendicular to an open end face having the through sections formed therein.
By means of such a construction, protrusions of complicated shapes can be formed by means of injection molding, thereby enabling formation of terminal electrodes of various patterns.